1) Field of the Invention
The present invention relates to an image display device that displays an image by changing a light-emission time according to display luminance.
2) Description of the Related Art
In a drive circuit of the image display device using an organic light emitting diode (OLED), the drive circuit in which light-emission luminance of the OLED is not changed but a light-emission time of the OLED is changed has been proposed in order to realize display luminance in each pixel. That is, in any pixel, the light-emission time of the OLED is lengthened when high intensity display is performed, and the light-emission time is shortened when low intensity display is performed.
As shown in FIG. 9A, the conventional image display device includes an OLED 101, an inverter 102 whose output terminal is connected to an anode side of the OLED, a thin film transistor 103 that functions as a switching element for resetting the inverter 102 by causing an input terminal and the output terminal of the inverter 102 to conduct, a data line 104 that supplies data potential according to the display luminance and sweep potential required during light emission as described later, and a capacitor 105 that is arranged between the data line 104 and the inverter 102. The inverter 102 is formed by a p-type thin film transistor 106 and an n-type thin film transistor 107. Specifically, drain electrodes of the thin film transistors 106 and 107 are connected to each other to form the output terminal, and gate electrodes of the thin film transistors 106 and 107 are connected to each other to form the input terminal. While a source electrode of the thin film transistor 107 is grounded, the source electrode of the thin film transistor 106 is connected to a power supply line 109 through an n-type thin film transistor 108.
FIG. 9B is a time chart that depicts potential sweeps during the operation of the conventional image display device shown in FIG. 9A. As shown in FIG. 9B, the operation of the conventional image display device is divided into an address period in which the data potential is written according to the luminance and a light-emission period in which light is emitted based on the data potential written. In the address period, the write of data potential Vdata from the data line 104 and a process of resetting the inverter 102 are simultaneously performed, and a difference (Vdata−Vres) between the written potential Vdata and potential Vres given to the input terminal of the inverter 102 by the reset process is generated between electrode plates of the capacitor 105.
In the light-emission period, the sweep potential having a triangle waveform is supplied from the data line 104, and the potential at the output terminal of the inverter 102 is larger than the reset potential Vres in the period in which the sweep potential is lower than the data potential Vdata. The OLED 101 is emits the light in the period, so that the OLED 101 emits the light only for a time according to the data potential Vdata supplied from the data line 104.
However, because the conventional image display device using the OLED has a configuration in which the inverter 102 is included, there are problems in that production becomes complicated and electrical power consumption is increased. These problems are explained below.
As shown in FIG. 9A, the inverter 102 has the configuration in which the p-type thin film transistor 106 and the n-type thin film transistor 107 are included. When the thin film transistors having the different conduction types are formed on the same substrate, because it is necessary to produce the thin film transistors through the different processes, there are problems in that production processes becomes complicated and production cost is increased.
Further, in the conventional image display device, as described above, when the data potential is written by the data line 104, it is necessary to perform the reset process by establishing a short circuit between the output terminal and the input terminal of the inverter 102 with the thin film transistor 103. The power consumption required for the reset process reaches 15% of all the power consumptions required for drive of the image display device, which prevents the decrease in power consumption.